3-D volume imaging has proved to be a valuable diagnostic tool that offers significant advantages over earlier 2-D radiographic imaging techniques for evaluating the condition of internal structures and organs. 3-D imaging of a patient or other subject has been made possible by a number of advancements, including the development of high-speed imaging detectors, such as digital radiography (DR) detectors that enable multiple images to be taken in rapid succession.
Cone beam (CB) computed tomography (CT) (CBCT) or cone beam CT technology offers considerable promise as one type of diagnostic tool for providing 3-D volume images. Cone beam CT systems capture volumetric data sets by using a high frame rate digital radiography (DR) detector and an x-ray source, typically affixed to a gantry that rotates about the object to be imaged, directing, from various points along its orbit around the subject, a divergent cone beam of x-rays toward the subject. The CBCT system captures projections throughout the rotation, for example, one 2-D projection image at every degree of rotation. The projections are then reconstructed into a 3D volume image using various techniques. Among the most common methods for reconstructing the 3-D volume image are filtered back projection approaches.
Although 3-D images of diagnostic quality can be generated using CBCT systems and technology, a number of technical challenges remain. In some cases, for example, there can be a limited range of angular rotation of the x-ray source and detector with respect to the subject. CBCT Imaging of legs, arms, and other extremities can be hampered by physical obstruction from a paired extremity. This is an obstacle that is encountered in obtaining CBCT image projections for the human leg or knee, for example. Not all imaging positions around the knee are accessible; the patient's own anatomy prevents the radiation source and image detector from being positioned over a portion of the scan circumference.
To illustrate the issues faced in CBCT imaging of the knee, the top view of FIG. 1 shows the circular scan paths for a radiation source 22 and detector 24 when imaging the right knee R of a patient as a subject 20. Various positions of radiation source 22 and detector are shown in dashed line form. Source 22, placed at some distance from the knee, can be positioned at different points over an arc of about 200 degrees; with any larger arc, left knee L blocks the way. Detector 24, smaller than source 22 and typically placed very near subject 20, can be positioned between the patient's right and left knees and is thus capable of positioning over the full circular orbit.
A full 360 degree orbit of the source and detector is not needed for conventional CBCT imaging; instead, sufficient information for image reconstruction can be obtained with an orbital scan range that just exceeds 180 degrees by the angle of the cone beam itself, for example. However, in some cases it can be difficult to obtain much more than about 180 degree revolution for imaging the knee or other joints and other applications. Moreover, there can be diagnostic situations in which obtaining projection images over a certain range of angles has advantages, but patient anatomy blocks the source, detector, or both from imaging over that range.
For imaging the leg, one way around this problem is to arrange the patient in a pose such that the subject leg is extended into a CBCT scanning apparatus and the paired leg is supported in some other way or bent with respect to the subject leg, such as at a right angle. This is the approach used, for example, in the CT scanner device taught in U.S. Pat. No. 7,394,888 entitled “CT Scanner for Lower Extremities” to Sukovic et al. In the methods of the Sukovic et al. '888 disclosure, the other leg must either be lifted out of place or spread at a distance, or is relaxed while the subject leg is lifted out of place and extended into the scanner equipment. This arrangement can be particularly disadvantageous for a number of reasons. It can be helpful, for example, to examine the condition of a knee or ankle joint under the normal weight load exerted on that joint by the patient. But, in requiring the patient to assume a position that is not usually encountered in typical movement, the Sukovic et al. '888 apparatus may obtain an image when there is excessive strain, or insufficient strain, or poorly directed strain, on the joint.
Another issue with conventional approaches relates to imaging of a load-bearing extremity such as the human leg. Because of the inability to image the leg under a normal load, as the patient is in a standing position, various artificial ways of mimicking load conditions have been attempted. Such approaches have used various types of braces, compression devices, and supports. As one example intended to remedy the shortcomings of conventional imaging techniques, the Sukovic et al. '888 disclosure teaches simulating the normal loading of the leg by elevating the leg to a non-standing position, then applying an external force against the leg. However, it can be readily appreciated that while this type of simulation allows some approximation of load-bearing limb response, it can be inaccurate. The knee or ankle joint, under some artificially applied load and at an angle not taken when standing, may not behave exactly as it does when bearing the patient's weight in a standing position.
Another difficulty with the Sukovic et al. '888 apparatus and with other devices designed to address knee and lower leg imaging relates to poor image quality. For image quality, the CBCT sequence requires that the detector be up close to the subject and the source of the cone beam radiation be at a sufficient distance from the subject. This provides the best image and reduces image truncation and consequent lost data. Positioning the subject midway between the detector and the source, as Sukovic et al. '888 apparatus and with other devices require, not only noticeably compromises image quality, but also places the patient too near the radiation source, so that radiation levels are considerably higher. One example of this strategy is shown in German patent publication DE 10146915. With the C-shaped gantry arrangement shown, centering the subject at the center of rotation of source and detector would apply considerably higher radiation amounts with each projection and severely compromise image quality. Any other positioning of the subject, such as closer to the detector, might reduce radiation levels over some part of the image capture sequence, but would result in unduly complex image reconstruction problems, since this would actually vary the distances between radiation source and subject and between subject and detector with each projection image obtained. Attempted imaging of the knee with such a system would require the patient to be supported in some way, balancing on the leg being imaged. It can be appreciated that this requirement is unreasonable or impossible for many situations in which an injured knee is being imaged. Thus, the C-shaped gantry shown would not be suitable for imaging only one knee of the patient.
Imaging of the foot and ankle presents additional obstacles for CBCT projection image capture. Approaches such as that given in the Sukovic et al. '888 disclosure, centering the foot between source and detector, suffer from the same problems of poorly positioned exposure and noticeably compromised image quality.
In summary, for extremity imaging, particularly for imaging the lower paired extremities, a number of improvements are needed, including the following: (i) improved placement of the radiation source and detector to provide acceptable radiation levels and image quality throughout the scanning sequence; (ii) system flexibility for imaging at different heights with respect to the rotational axis of the source and detector, including the flexibility to allow imaging with the patient standing or seated comfortably, such as with a foot in an elevated position, for example; (iii) improved patient accessibility, so that the patient does not need to contort, twist, or unduly stress limbs or joints that may have been injured in order to provide images of those body parts; (iv) improved ergonomics for obtaining the CBCT image, allowing the patient to stand with normal posture, for example. This would also allow load-bearing extremities, such as legs, knees, and ankles, to be imaged under the normal load exerted by the patient's weight, rather than under simulated loading conditions as taught in the Sukovic et al. '888 disclosure and elsewhere.
Thus, it can be seen that although a number of solutions have been proposed to address the problem of CBCT extremity imaging, conventional solutions fall short of what is needed for both usability and performance.